Corrosion-resistant aluminum alloy articles and method of making the same



nese 0.6; (B)same as Patented June 28, 1932 UNITED STATES PATENT OFFICEEDGAR H. DIX, JR., 0]! OAKHONT,

PENNSYLVANIA, ASBIGNOB TO ALUIIHUM comm VAJTIA- CORROSION-RESISTANTALUMINUM ALLOY ARTICLES AND IMHO!) 01' MAKING m BAKE 1T0 Drawing.

Aluminum base alloys are for many purposes superior to pure aluminum butat least some of them are subject to more or less rapid deterioration byreason of corrosion or like action, which militates against their use insituations where they are exposed to agencies conductive to or producinsuch effects. Impure aluminum is also a acted by the same agenciesthough eiierally to a less extent. Among the most 1m ortant and valuablealloys are those of higli tensile strength, as for example alloyscontaining copper, magnesium, silicon, manganese, and zinc, in variouscombination and proportions as components in addition to aluminum. Ofthese I may mention specifically the well known alloys containing one ormore of the above-named elements in up roximately the followingpercentages, an usually containing small amounts of impurities, as forexample iron: (A)-copper 3 to 5, magnesium 0.5, manga- (A) but withoutmagnesium and with copper 45.5; (C)- magnesium 0.7, silicon 0.9.

The high-strength alloys, including the three specifically mentionedabove, require heat treatment to bring out their desirable physicalproperties to the fullest extent. The

eat treatment usually consists in heating the finished article to atemperature of around 500 C. for a suitable period and then uenching,followed by aging. Alloy described above, is a ed at room temperature,but alloys (B) an (C) may be aged-(after quenching at a temperaturebetween 100 0., and 150 approximately. This is known as artificialaging. Alloys (B) and (C) have greater resistance to corrosion beforeaging than after, and experience in general indicates that artificial orhigh temperature aging decreases resistance.

The deterioration of alloys by corrosion is a serious matter in variousarts, especially where replacement of corroded parts would mean virtualrebuilding, or where failure of even a single part may mean disaster, asfor example in aircraft. Moreover, the deterioration does not alwaysbetray itself by excessive surface corrosion, but may be and notinfrequently is due to internal or inter-granu- Appllcation med January82, 1987. Serial Ho. 169,880.

lar action, leaving the surface of the part with no more than the eiipected amount of oxidation or the like. he result is that metal which toall appearances is unharmed ma nevertheless be seriously weakened, thusma ing it diflicult to detect imperfect parts.

I have accordingly been led to devise my present invention, which hasfor its chief object to protect the impure aluminum or aluminum alloyarticle against corrosion by a non-porous surface layer or coating ofsubstantially pure aluminum or of corrosion-resistant aluminum alloy.This object can be attained by casting the alloy or impure metal againstthe protective aluminum, preferably in the form of sheets or plates, andsubjecting the'ingot thus formed to pressure and heat. By such methodthe coating and the underlying metal can be bonded together with atenacity exceeding the tensile strength of the coating.

In practicing the process in the preferred manner it is convenient toarrange t e aluminum plates or sheets in the form of a lining for themold in which the ingot or other article is to be cast, but it is notnecessary, and indeed it is not in neral desirable, to have all innersurfaces 0 the mold lined with the alumium plates. On the contray itusually is sufiicient and in most cases advisable to line only twoopposite surfaces. I have found that if an ingot so surfaced is rolleddown to a thin sheet or plate, say a sixteenth of an inch thick, thealloy exposed at the edges in many cases is not appreciably subject toweakening corrosion, if at all, even under severe conditions. Apparentlythe pure aluminum with which the alloy is surfaced exerts a protectiveaction on the uncovered ed s. On the other hand, in casting ingots orillets for drawing tubes, rods, wire etc., the entire inner surface ofthe mold ma have a lining in the form of a tube{ which is to beconsidered the equivalent 0 one or more plates or sheets.

In order that the ingot may be rolled down or otherwise shaped byrolling, drawing, or like operations, the castin of the alloy againstthe aluminum moldmn must prw duce an initial bond which wi withstanshearing or similar stresses produced in such operation tending to causethe surface plates to slip on the underlying allo To insure this initialbond without harm y affecting the surfacing sheets against which thealloy is cast it is advisable to observe certain precautions.

In the first place the tendency of the molten alloy to fuse or dissolvethe aluminum sheets should be restricted so that the fusion or solutionproduced is only superficial. A certain amount of diffusion of one metalinto the other is often desirable and in some cases essential, but thisdiffusion can be produced by subsequent heating and working of theingot. and if the initial diffusion or penetration of alloy-componentsextends too close to the outer surface of the aluminum sheets or platesthe subsequent difl'usion, combined with the thinning of the protectivealuminum layer by working, may in effect result in converting such layerinto a corrodible alloy. While care in rolling or other working. andavoidance of too high temperatures in heating the ingot for working,will aid in preventing the result alluded to, care in such operations isnot always suflicient and hence the casting operation should be properlycarried out. It is therefore desirable to use a mold which will conduct,absorb, or dissipate heat rapidly, and the lining sheets should be inclose contact therewith, so that the heat imparted to the sheets orplates by the larger body of molten alloy, preferably at a. temperatureas low as possible consistent with proper casting and initial bonding,will flow to the mold walls too rapidly to permit more than surfacefusion of the aluminum. Iron molds having smooth inner surfaces havebeen found satisfactory for the purpose, and these are referablywater-cooled Even with molds of the kind just described it is advisableto use lining plates of substantial thickness not only because too thina plate is more easily melted or raised to a dissolving or diffusingtemperature but also if the plate is not of suflicient thicknessoriginally' the subsequent reduction in thickness by working may make ittoo thin to afford the desired degree of protection.

It is also advantageous, in casting the ingot, to avoid pouring themolten alloy against any part of the mold lining, as it has been foundthat a stream of molten alloy flowing on the aluminum plate is apt tomake a hole or a cavity therein. Such effect may be due in part, atleast, to erosion or some analogous action. Avoidance of splashing. ofthe molten alloy is also desirable. In general I try to have the alloyfold itself, so to speak, into contact with the plates, as a liquidpoured gently into a vessel rolls or folds into contact with the walls.Then if there is any substantial fusion of the aluminum plate the fusedportion is not moved out of of lace, and deeper or more extensive fusionis t erefore less likely to occur. A good method of casting is to pourthe molten alloy in a gentle stream down an unlined surface of the mold,tilting the mold at first so as to incline the unlined surface andgradually straightening it up as the pouring proceeds. In this way, withthe temperature of the alloy, the mass of the alloy and the mass of thelining sheets, and the rate of pourin so related that the alloy freezesagainst t e lining surface at about the same rate as its contacttherewith, the disadvantageous conditions mentioned can be avoided. WhatI aim at is to produce a union which is predominantly one of cohesion asdistinguished from adhesion.

Contrary to expectation, my observation is that the surface oxidationordinarily found on aluminum does not prevent the necessary initialbonding, and hence, although dirt and grease should, obviously, beremoved, it has not been necessary to clean the aluminum by etching withacid or alkali, or by sand-blasting or wire-brushing, or the likeabrasion. It is probable that this oxide film is one cause of thefailure of attempts to bond plates of aluminum and aluminum allowtogether by hot-rolling of the two in contact. I believe that in myprocess the molten alloy penetrates the oxide film at many points andthereby comes into cohesive or molecular contact with the metallicaluminum, and may, conceivably, dislodge or penetrate under some of theoxide particles. In strengthening this initial bond or union by rollingor other working, the numerous polnts or minute areas of cohesion areextended or spread out, but intervening oxide particles are not. Hencein the aggregate the area of molecular cohesion so greatly exceeds theaggregate area of the island-like noncohering points that the latter arein effect non-existent. Whether the theory just outlined be true or not,a union comparable to such a bond in completeness and tenacity isobtainable when my process is carried out with care and intelligence.

The coating plates may be held against the inner surfaces of the moldclamps, or they may have their edges seated in grooves. Making thelining in two or more parts is advantageous in that it permits expansionwith less or no tendency to buckling or warping during heating by thehot alloy. In many cases annealing the coating material minimizes suchtendency.

The initial bond of the surfacing plates to the alloy or impure aluminumcore is made complete by heat and pressure, preferably rolling pressure,repeated as may be necessary to bring about complete union and, ifdesired, reduce the ingot or billet to a plate or sheet of the desiredthickness. In such treatment there is more or less diffusion ofalloycomponents into the aluminum plates, and

by means ofnaeaooo care should be taken not to cause or permit thisalloying action to extend too close to the outer surface. It istherefore desirable in most cases to kee the working temperature belowthat at w ich too rapid solution of copper or other non-aluminous metalor metals resent takes place. For this reason, especially in the case ofalloys whose physical properties are to be improved by heat-treatment,it is desirable that the temperature to which it is heated for rollingor other working should be well below the melting point of the eutecticor eutectics. For example, with. an alloy of type A above referred tofor which a heat-treating temperature of 520 C. is satisfactory. aworking temperature between 480 and 500 C. gave perfect results, thesurface sheets or layers being so completely united to the alloy thatthey could not be stripped off nor could separation be even started. Inthis example an iron mold was used, 8 inches deep, 7 inches wide, and1.5 inches from front to back. inside measurements. .The side walls wereinch thick, edge walls 1 and bottom 3 inches thick. The lining sheets,smooth on both sides and 0.1 inch thick, were of annealedelectrolytically refined aluminum 99.95 per cent pure, and were cut toallow for the expansion incident to the heat to which they weresubjected in casting the alloy. The alloy contained copper 3.33 percent. silicon 0.33 per cent, magnesium 0.49 per cent, manganese 0.40 percent, iron 0.34 per cent, and the rest aluminum. The alloy, at atemperature of about 730 C., was poured into the open top of the mold inthe ordinary manner. After removal from the mold the ingot was heated to480 to 500 C. and while at this temperature was rolled to 0.5 inchthickness. It was then reheated to 400 C. and rolled to 0.1875 inch. Thesheet was then annealed at 250 C. and cold-rolled to 0.064 inch, andfinally heat-treated by holding it for twenty minutes in a fused bath ofsodium and potassium nitrate at 520 C., followed bv quenching in coldwater and aging at room temperature.

The surface appearance of the sheet was better than that of a sheetproduced according to the usual method and there was no evidence ofblistering or like defects. Microscopic examination showed there were nodiscontinuities between the alloy and the pure aluminum coating, andindicated perfect union. Diffusion of copper from the alloy into thealuminum coating, 0.0045 inch thick, was slight but easily seen.

After aging for one week, five specimens of the sheet were tested forcorrosion, along with similar pieces of the same alloy uncoated, bycontinuous subjection for eight weeks to a mist produced by spraying a20 per cent solution of common salt. At the end. of this time thetensile strength and elongation of the coated articles had suffered noimpairment. On the other hand the average of the five uncoated specimensshowed a loss of 13.6 per cent in tensile strength, or more than 4 tonsper square inch, and the elongation had decreased from 20.4 per cent to7.0 per cent in 2 inches, or a loss of 65.7 per cent. Larger ingots oftype A alloy, approximately 4 by 12 by 24 inches in size, with surfacingsheets inch thick, have also been cast and worked with completelysatisfactory results. I have also found that fabrication of coatedingots into other articles is notably easier and can be effected at lesscost than with uncoated ingots of the same alloy, as the coatingeliminates or greatly decreases many of the difliculties hithertoencountered in rollmg.

In speaking of heating and working the ingot after casting I do not meanto imply that the ingot must be allowed to cool below the workingtemperature and then reheated. On the contrary it is only when thecasting has cooled too far that reheating need be resorted to, and insome cases it is advantageous to begin the working without allowing thecasting to cool far enough to necessitate reheating. This is especiallytrue with alloys of high magnesium content. or, in general. with alloyswhich oxidize readily. With such alloys, it appears that if the core andthe coating sheets do not stick at all points air finds its way inbetween, with the result that in the reheating the oxide or nitridefilm, or whatever it may be, is materially increased and becomes toogreat to permit eventual attainment of a bond complete enough to preventseparation. l have found, however. that if the ingot be subjected topressure. say a light pass between rolls. before it has cooled too far,preferably as soon as it can be taken from the mold and before it hascooled appreciably, the union can be improved to such an extent thatadequate initial bond can be obtained.

I have found that alloys containing copper, say from 2 to 6 per cent,are most suitable for my process. On the other hand magnesium tends tobe disadvantageous and apparently should be less than about 1 per centto give in the casting step alone the bond needed to prevent excessiveoxidation or the like between the alloy and the aluminum surfacingplates. but by pressing the plates firmly on the alloy core before theingot has cooled down too much, preferably applying the pressure byrolling as soon as the ingot can e taken from the mold, as describedjust above, the desired bond can be obtained with alloys containing muchhigher amounts of magnesium. For the surfacing plates I prefer the highpurity electrolytically refined aluminum now available on the market butordinary commercial aluminum may be used, as may also aluminum alloyshaving adequate resistance to corrosion, and in the claims the termaluminous metal is intended to include commercial aluminum andcorrosionresistant aluminum alloys as well as aluminum of high purity.The alloys are generally harder than aluminum and are thereforeadvantageous where a harder surface is desired. Alloys of aluminum andmanganese, and aluminum and beryllium, are suitable for such purpose.

I am aware that it has bee proposed, in British Patent No. 25,380 of1898, and in Swiss Patent No. 18,292 of 1899, to provide an article ofaluminum alloy with a coating of pure aluminum by rolling the aluminumand the alloy together, hot or cold; but such method has been found tobe unsuccessful, it being impossible to produce thereby the cohesivebond whichcharacterizes the product of my process and'makes the twometals, the alloy and the aluminum, an integral unit. The lack ofcohesion between the two bodies in the product of the method referred tois evidenced by the fact that if the article so made is heated thealuminum layer pufi's up in the form of blisters, indicating thatalthough the two layers may have been close together they were 1n factnot in actual molecular metal-to-metal contact.

It is to be understood that the invention is not limited to the specificprocedure herein described but can be carried out in other ways withoutdeparture from its spirit. Nor it is confined to alloy, strictly socalled, as the coated metal or metal to be coated, since it may, ingeneral, be employed with impure aluminum as the equivalent of an alloyand the subjoined claims are intended to be so interpreted.

I claim- 1. Process of coating corrodible aluminum alloy withcorrosion-resistant aluminous met- :11. comprising casting a body of thealloy in contact with a plate of the aluminous metal while dissipatingthe heat of the alloy at a rate adapted to cause freezing of the alloyon the plate with not more than superficial fusion of the plate, wherebyan initial surface-bond between the two is produced, and completing thebond by heat and pressure with the article at a temperature suificientlybelow the eutectic melting point to prevent excessive intor-diffusion.

2. Process of coating corrodible aluminum alloy with corrosion-resistantaluminous metal, comprising casting a body of the alloy in contact witha plate of the aluminous metal while dissipating the heat of the alloyat a. rate adapted to cause freezing of the alloy on the plate andproduce an initial bond with not more than superficial fusion of thecontiguous plate surface, hot-working the article to complete the bondwithout causing suflicient inter-difl'usion to bring non-aluminous alloycomponents to the surface of the alumiworked article.

3. Process of coating corrodible aluminum alloy with corrosion-resistantaluminous metal, comprising lining with the aluminous metal a moldhaving thermally conductive walls, casting therein a body of the alloyand by regulation of the alloy temperature and pouring rate causing thealloy to freeze on the mold lining with cohesion-contact without morethan superficial fusion of the lining, hot-working the article at atemperature sufliciently lower than the eutectic melting point toprevent such diffusion of non-aluminous alloy-components into thecoating metal as to destroy the corrosion-resistant repertiels thereof,and heabtreating the wor ed artic e.

4. Process of coating corrodible aluminum alloy with corrosion-resistantaluminous metal, comprising casting a body of the alloy against a plateof the aluminous metal without erosive movement of the alloy thereon andby regulation of the temperature and pouring rate according to the rateof heat dissipation from the contiguous surface of the plate causing thealloy to freeze on said surface and produce an initial bond with notmore than superficial fusion, completing the bond by hot-working, andheat-treating the worked article. a

5. Process of coating corrodible aluminum alloy with corrosion-resistantaluminous metal, comprising lining a metal mold with the aluminousmetal, casting in the lined mold a body of the aluminum alloy and byregulation of the temperature and pouring rate of the alloy according tothe rate of heat transfer through the mold lining and the mold wallscausing the alloy to freeze in cohesive contact with the lining with notmore than superficial fusion thereof whereby an initial bond isproduced, hot-working the article to complete the bond, andheat-treating the worked article.

6. Process of coating corrodible aluminum alloy with corrosion-resistantaluminous metal. comprising casting against a plate of the aluminousmetal a bodv of aluminum alloy containing copper and by regulation offactors determinative of the solidification of the alloy causing thesame to freeze on the plate and produce an initial bond with not morethan superficial fusion thereof, completing the bond by hot-working at atemperature too low for excessive difi'usion of copper into the coatingmetal, and heat-treating the worked article.

7. Process of coating corrodible aluminum alloy with corrosiomresistantaluminous metal, comprising casting a body of aluminum alloy containingcopper against a lining of the aluminous metal in a mold havingheatconducting walls and by regulation of other casting conditionscausing the alloy to freeze tic to prevent excessive penetration ofcopper into t e aluminous coating metal.

8. Process of coating corrodible aluminum alloy with corrosion-resistantaluminous metal comprising casting a body of the alloy against a plateof the aluminous metal and by regulation of the casting conditionscausing the alloy to freeze on the plate and produce an initial bondwith not more than superficial fusion of the plate, and hot-working thearticle under conditions preventing destruction of thecorrosion-resistant properties of the aluminous metal by diffusion ofnon-aluminous alloy components into the same.

9. Process of coating corrodible aluminum alloy with corrosion-resistantaluminous metal, comprising casting a body of the alloy against a plateof pure aluminum and by regulation of the casting conditions causing thealloy to freeze on the aluminum and produce an initial bond with notmore than superficial fusion of the aluminum, completing the bond byhot-working under conditions preventing destruction of thecorrosion-resistance of the aluminum by diffusion of non-aluminous alloycomponents into the aluminum, and heat-treating the worked article.

10. Process of coating corrodible aluminum alloy withcorrosion-resistant aluminous metal, comprising casting a body ofaluminum alloy containing copper against a late of pure aluminum and byregulation 0 the casting conditions producing an initial bond betweenthe alloy and the aluminum with not more than su erficial fusion of thelatter or penetration 0 copper into the same, hotworking the article ata temperature too low for penetration of copper far enough to destroythe corrosion-resistant properties of the coating, and heat-treating theworked article.

11. Process of coating corrodible aluminum alloy withcorrosion-resistant aluminous metal, comprising casting against a plateof pure aluminum a body of aluminum alloy containing copper andmagnesium and by regulation of temperature and pouring conditionsaccording to the rate of conduction of heat from the surface of thealuminum causing freezing of the alloy on the aluminum and producingthereby an initial bond with not more than superficial fusion of thealuminum plate or penetration of copper into the same, completing thebond by hot-working the article under conditions preventing destructionof the corrosion-resistant properties of the aluminum by penetration ofcopper into the latter, and heat-treating the worked article.

12. A corrosion-resistant aluminous metallic article comprising arelatively thick main body of corrodible aluminum alloy and a relativelythin wrought non-porous protective coating of corrosion-resistantaluminum directly cohesively united with the main body, the articlebeing capable of being heat treated without blistering.

13. A corrosion-resistant aluminous metallic article comprising arelatively thick main bod of heat-treated aluminum alloy and a reatively thin wrought non-porous protective coating of annealedcorrosionresistant aluminum directly cohesively united with the mainbody, the article being capable of being heat-treated withoutblistering.

14. A corrosion-resistant aluminous metallic article comprising arelatively thin main body of heat-treated aluminum alloy containingbetween 2 and 6 per cent of copper, and a relatively thick wroughtnonporous protective coating of annealed corrosion-resistant aluminum,directly cohesively united with the main body and free from blisters.

15. A corrosion-resistant aluminous metallic article comprising arelatively thick main body of heat-treated aluminum alloy containingbetween 2 and (3 per cent of copper, and a relatively thick wroughtnon-porous protective coating of annealed corrosionresistant aluminum,directly cohesively united with the main body and free from blisters,said article being capable of withstanding continuous contact with mistfrom a 20 per cent salt solution for a period of at least'eight weekswithout appreciable loss of tensile strength and ductility.

16. An article composed of a relatively thick body of corrodiblealuminum alloy having a relatively thin wrought non-porous coating ofcorrosion-resistant aluminous metal directly cohering with a tenacityexceeding the tensile strength of the coating.

17. An article composed of a relatively thick body of corrodiblealuminum alloy having a relatively thin wrought non-porous coating ofcorrosion-resistant aluminous metal directly cohering with a tenacityexceeding the tensile strength of the coating, said article beingcapable of withstanding continuous contact with mist from a 20 per centsalt solution for a period of at least eight weeks without appreciableloss of strength and ductility.

18. A corrosion-resistant aluminous metallic article comprising arelatively thick main body of heat-treated aluminum alloy and arelatively thin wrought-non-porous protective coating of annealedcorrosionresistant aluminum, directly cohesively united with the mainbody and free from blisters, said article being capable of withstandingcontinuous contact with mist from a 20 per cent salt solution for aperiod of at least eight weeks without appreciable loss 1,ses,oae

June 28, 1932.

page 2, line 93, for "allow" read it; page 5, line 85, claim 14, for

nt. should be read with these corto the record of the case in the M. J.Moore,

of strength and ductility.

In testimony whereof I hereto aflix my signature.

EDGAR H. DIX, JR.

CERTIFICATE or CORRECTION.

PatentNo. 1,865,089.

EDGAR H. DIX, JR.

it is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 1,line 79, for the misspelled word "contray" read contrary; alloy; page 4,line 32, for "it is" read is "thin" read thick. and lines 88 and 97,claims 14 and 15 respectively for "thick" read thin; and that the saidLetters Pate rections therein that the same may conform Patent Office.

Signed and sealed this 25thday of October, A. D. 1932.

(Seal) Acting Commissioner of Patents.

blisters, said article being capable of withstanding continuous contactwith mist from a 20 per cent salt solution for a period of at leasteight weeks without appreciable loss 1,ses,oae

June 28, 1932.

page 2, line 93, for "allow" read it; page 5, line 85, claim 14, for

nt. should be read with these corto the record of the case in the M. J.Moore,

of strength and ductility.

In testimony whereof I hereto aflix my signature.

EDGAR H. DIX, JR.

CERTIFICATE or CORRECTION.

PatentNo. 1,865,089.

EDGAR H. DIX, JR.

it is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 1,line 79, for the misspelled word "contray" read contrary; alloy; page 4,line 32, for "it is" read is "thin" read thick. and lines 88 and 97,claims 14 and 15 respectively for "thick" read thin; and that the saidLetters Pate rections therein that the same may conform Patent Office.

Signed and sealed this 25thday of October, A. D. 1932.

(Seal) Acting Commissioner of Patents.

